A major difficulty for cosmetic emulsions (and for hard surface waxes and polishes) is the fact that the emulsifier is deposited on the surface along with the oil (or wax) phase when an emulsion product is applied. This results in the oil (or wax) phase being readily re-solubilized or emulsified back into water by the deposited emulsifier when the surface is wetted. Thus, the benefits achieved by applying the emulsion product in the first place, can be greatly diminished by rewetting. For example, sunscreen actives can be readily washed off by bathing following the application of a sunscreen lotion. In order to minimize this effect, in the past, many sunscreen lotions were formulated as water-in-oil (w/o) emulsions. These types of emulsions are made with emulsifiers such as cholesterol, lanolin, lanolin alcohol, low HLB emulsifiers and calcium or magnesium fatty acid soaps. These materials are poorly water soluble and, when deposited on skin as part of an emulsion, they have very little ability to re-emulsify the oil phase when the skin is wetted. These formulations, however, tend to feel oily or greasy upon application and often do not dry (or rub-in) to a cosmetically acceptable non-oily film. In addition, they can be fairly difficult to formulate and making a stable w/o lotion or a low viscosity w/o emulsion can be quite a challenge.
In addition to w/o emulsions, polymeric emulsifiers that make oil-in-water (o/w) emulsions are often used to reduce the ability of the oil phase material to re-emulsify. The commonly used polymeric surfactants precipitate (or “plate out” as it is termed) on the skin. This deposition onto the skin is sufficient to reduce the ability of the polymeric surfactant to re-dissolve in water. Further, most of these polymeric surfactants have low surface activity and do not promote the rewetting of the deposited oil phase material by applied water. This combination of effects is used to produce cosmetically acceptable creams and lotions that have a reduced tendency to wash off. Such formulations find use in water resistant or water proof sunscreen products.
While the use of polymeric surfactants can produce cosmetically acceptable products with very good water resistance, the feel of these products is not always desirable because, upon application, the polymers can feel slimy and the emulsion can take too long to “rub-in”. Therefore, the application feel of emulsions made with polymeric surfactants is often very different from that of a conventional soap based o/w emulsion which is the usual type of rub-in feel that is expected by consumers.